ONCOGENE-SUPPRESSOR AND TUMOR-SUPPRESSOR GENE-RELATED PROTEINS IN PLANTS AND FUNGI

Protooncogene- and tumor-suppressor gene proteins serve essential func tions in the regulation of proliferation and differentiation of cells. Abnormal regulation or mutation of these genes, or transformation wit h retroviral homologs, may lead to tumor development in animals. In co ntrast to vertebrates, only few data on these genes exist in plants an d fungi. Plant nuclear protooncogene homologs, such as myb and myc hav e multiple regulatory functions in metabolic pathways not existing in mammalian cells; they are involved in the complex regulation of anthoc yanin (purple pigment) and phlobaphene (red pigment) biosynthesis, lig nin production, trichome differentiation, dehydration stress gene expr ession and seed development. Apart from these well-characterized roles in plant-specific pathways, few experimental data have been reported on a functional significance in growth and development. A screening fo r nuclear protooncogene- and tumor-suppressor gene-related proteins in the myxomycete Physarum polycephalum revealed the existence of homolo gs of vertebrate c-myc, c-fos, c-jun, p53, and retinoblastoma proteins during the synchronous cell cycle or sclerotization. The p53 homologs of Physarum and Zea mays were shown to be specific for quiescent stag es of their life cycles. Plants and lower eukaryotes, such as fungi, m ay be useful experimental systems to elucidate novel functions of prot ooncogene- and tumor-suppressor proteins in cell cycle regulation and development, or to reveal target genes that might be difficult to iden tify in complex mammalian systems. Recent data indicate that oncogenes and tumor suppressors in animals have more cellular targets than orig inally proposed; some of these might be as unexpected as in plant seco ndary metabolism.